This invention relates to integrated circuit audio systems and particularly to integrated circuit audio systems useful in television receivers.
Conventional television audio signals comprise angle-modulated carrier waves, characterized in that information is carried by changes in the frequency or phase of the carrier wave, as distinct from changes in carrier wave amplitude. The advantages of frequency/phase modulation over amplitude modulation are well known and the circuitry for detection of such waves in television receivers is not only well known, but has been extensively used in integrated circuit form.
With the growing emphasis on use of integrated circuitry in electronic apparatus and especially in mass produced television receivers, circuit designs that minimize usage of components and circuitry that cannot conveniently be incorporated into an integrated circuit format are in great demand.
U.S. Pat. No. 3,519,944 issued July 7, 1970 in the name of Jack Avins discloses an integratable audio circuit that has found wide acceptance and use in the television industry. In that circuit, the intercarrier sound signal is limited to remove amplitude modulation noise and supplied to a discriminator. The discriminator, in turn, supplies a differential amplifier detector through a single, external phase shift network which is series-parallel resonant at frequencies above and below the 4.5 MHz intercarrier sound beat frequency. Since signals across the phase shift network are in antiphase, that is, they are approximately 180.degree. out of phase with each other, they subtract in the output of the differential amplifier detector further reducing variations in carrier amplitude. The result is a fairly linear detector detection curve with signal deviation. The circuit is very highly integrated with the exception of the parallel tuned circuit elements.
Recently a number of different broadcast transmission systems have been proposed to enable television signals to include a second sound channel for stereo and to have a third channel capability for a foreign language. In all of the proposed systems, the signal handling capability will impose more stringent requirements on the linearity of the audio circuitry. The linearity of the patented system is not believed satisfactory in this respect since the present deviation of .+-.25 KHz will need to be increased substantially to produce sound quality compared with non-television stereo sources. Thus, present integrated circuit designs for monaural television sound must be revised to provide comparable performance with proposed television multiple sound transmissions.
While the technology is readily available to yield excellent wide-band audio response in television receivers, different circuitry is required. This poses a dilemma for manufacturers desirous of keeping costs at a minimum without sacrificing performance. It would be very advantageous to enable the continued use of presently available integrated audio circuitry even in "top-of-the-line" television receivers having the added sound service capability circuitry. Thus there is a need in the art for a simple circuit to enhance the capabilities of the above-mentioned integrated audio circuit.